1. Field of the Invention
The invention relates to a signalling gauge and more particularly to a magnetically- activated signalling gauge where the visual indication of the reading is not affected by a magnetic field of an actuating magnet.
2. Description of the Related Technology
Signalling gauges are instruments or meters which are utilized to monitor various parameters such as temperature, pressure, liquid levels or various electric properties. These instruments are specifically configured to give an indication when the monitor condition reaches some predetermined point.
In prior signalling gauges, the predetermined point is typically detected by electromechanical contacts. One contact is located on the reading pointer of the meter. A second contact may be either fixed or adjustable and located in the travel path of the pointer contact. U.S. Pat. Nos. 3,586,799 and 4,021,627, the disclosures of which are expressly incorporated herein, show electromechanical contact signalling gauges. (Furthermore, various gauges of this type have been sold under the registered trademark SWICHGAGES by Frank W. Murphy MFR., Inc.)
Electromechanical signalling gauges are quite adequate for a great number of purposes. They do, however, possess a number of shortcomings. The most significant shortcoming is the electrical contact integrity. The lifespan of electromechanical contacts is finite and depends on the number of operations in the environment to which they are exposed. Exposure to atmospheric conditions containing dirt, dust or various chemical gases can significantly degrade contact integrity and negatively impact on contact life span. As a result, frequent contact cleaning and/or replacement is necessitated.
A further drawback is the effect the mechanical contact has on the movement integrity. In order to establish or break electrical contact, a force significant to the operation and accuracy of the meter must often be applied to the indicating pointer. This precludes or significantly complicates the utilization of sensitive or balanced meter movements in signalling gauge applications. Electromechanical contacts are quite bulky and do not lend themselves to installation on aircoil or D'Arsonval movements such as those found in automotive instruments or many electrical condition monitoring instruments.
Another significant disadvantage of electromechanical contacts is the possibility of spark production. According to the American Petroleum Institute, Class I Division II environments classified as hazardous may not contain electrical contacts which are capable of producing a spark. Such a contact may result in ignition of flammable gases which may be present. Standard electric or electromechanical contact signalling gauges must be isolated from this environment by either large, expensive explosion proof enclosures or electronic barriers.
A final disadvantage of conventional electromechanical contacts is the inability to efficiently switch low level digital milliamp levels. Recommended operating levels for electromechanical contacts are significantly higher than conventional digital switching signal levels.
Other signalling gauges have utilized opto electronic sensors. This has not proved satisfactory in many applications because such gauges require hermetic sealing in order to operate for any period of time. Dust and dirt buildup significantly impairs the sensor, thereby rendering the gauge inoperative. The sealing provisions required add a significant expense to the production of the instruments.